1. Field of the Invention
The field of the invention is that of multiplexed data communication by radio and in particular half-duplex radio systems in which a mobile station cannot send and receive simultaneously.
The invention finds an advantageous application in cellular mobile radio systems such as the GSM system (Global System for Mobile communications). The invention has been developed in particular for the TransEuropean Trunk Radio (TETRA) system which is a PMR (Professional Mobile Radio) system for networks used by emergency services, taxis, etc.
2. Description of the Prior Art
In this type of system stable calls in the base station to mobile station direction (up link direction) or in the mobile station to base station direction (down link direction) are conveyed on traffic channels (TCH).
Digital radio systems usually operate in TDMA (Time-Division Multiple Access) mode, including TETRA type systems.
In TDMA mode each traffic channel is allocated a transmission frequency (or a set of frequencies in the GSM system, which uses frequency hopping) which is shared by time-division multiplexing between a plurality of calls with different mobile stations (typically four in the TETRA system or eight at present in the GSM system).
Time-division multiplexing entails dividing time into frames of fixed and predetermined duration, the frames being in turn divided into time slots. Each call is associated with one or more time slots. Thus a frame comprising N time slots can correspond to N calls, each mobile station communicating in one time slot of each frame. At present N=4 in TETRA systems and N=8 in GSM systems.
Each call can convey either data or speech signals coded digitally at a low bit rate (below 16 kbit/s).
A new generation of radio systems is currently undergoing standardization (in particular TETRA type systems), and in these systems it is proposed to substitute FDMA (Frequency-Division Multiple Access) for TDMA.
In FDMA mode each call between a base station and a mobile station is associated with a first frequency for the up link direction (from the mobile station to the base station) and a second frequency for the down link direction (from the base station to the mobile station). Operation is continuous in time rather than divided into time slots as in TDMA mode.
The combination of FDMA mode and half-duplex operation implies that a mobile station is either sending continuously (on a first frequency) or receiving continuously (on a second frequency).
Although continuous reception does not pose any particular problem, the same cannot be said for continuous transmission.
Continuous transmission rules out the transmission of signalling associated with the call, unless interruption of traffic is tolerated. By signalling associated with the call is meant, for example, instructions or parameters connected with the operation of the network and relating to the management of the traffic channel in use.
Also, there is no simple way to implement a "listen" function with continuous transmission. A "listen" function is very important, however, since it enables applications including:
listening to the signalling associated with the down link, for example to implement pre-emptive priority, i.e. the possibility (when all the mobile stations are to be asked to clear their respective channels in an emergency) of reaching not only the mobile stations which are receiving but also the mobile station which is sending, for example to send an APB to all vehicles in a common action group, PA1 detecting and/or measuring signals broadcast in adjacent cells in the case of a cellular network, for example to prepare for handover, i.e. the passage from one base station (one cell) to another; PA1 enabling each of two mobile stations which dialog directly with each other, without calls passing through the base station, to listen out in case the base station is attempting to reach it, for example to tell it that there is a more important call waiting (this is referred to in this document as "double-standby"). PA1 listening to down link signalling associated with the call; PA1 detecting and/or measuring the signals broadcast in adjacent cells, in the case of a cellular network; PA1 sending upward signalling associated with the call; PA1 receiving information directly from another mobile station. PA1 changing from send mode to receive mode during the start of said up link control frame; PA1 receiving at least one message contained in a down link control frame; PA1 at the end of said up link control frame, changing from receive mode to send mode or, where applicable, interrupting the call. PA1 the signal is organized in frames of predetermined fixed duration grouped into multiframes comprising a determined number of frames including at least one control frame, at least some of said control frames being listening frames during which the sending mobile station interrupts sending, changes to receive mode and then, if appropriate, returns to send mode.
Implementing a "listen" function is very simple in TDMA mode, in which a call is made up of a succession of time slots, with one time slot per frame. All that is required is to switch to receive mode between the time slots of the same call, in order to listen, and then to revert to send mode. Although the time to change from one mode (send or receive) to the other is relatively long, the time between two time slots assigned to the same call, which is the duration of N-1 time slots in the case of a frame with N time slots in total, is always more than enough.
Obviously in the case of half-duplex FDMA transmission the fact that the mobile station sends continuously rules out the solution used in TDMA mode.
One object of the invention is to solve the various problems that arise on changing from TDMA mode to FDMA mode in a half-duplex radio system.
To be more precise, one object of the present invention is to provide a half-duplex FDMA digital radio system which enables a mobile station communicating with a base station to transmit signalling associated with a call without serious disturbance of traffic.
Another object of the invention is to provide a system of this kind enabling a mobile station in send mode to use a "listen" function, in particular to listen out for down link signalling, to detect and/or measure signals broadcast in adjacent cells, or to implement "double-standby".